1. Field of the Invention
The present invention relates to gold powders and to methods for producing such powders, as well as intermediate products and devices incorporating the powders. In particular, the present invention is directed to powder batches of gold metal particles with a small average particle size, well controlled particle size distribution, spherical morphology and high crystallinity.
2. Description of Related Art
Many product applications require metal-containing powders with one or more of the following properties: high purity; high crystallinity; small average particle size; narrow particle size distribution; spherical particle morphology; controlled surface chemistry; reduced agglomeration of particles; and high density (low porosity). Examples of metal powders requiring such characteristics include, but are not limited to, those useful in microelectronic applications, such as for conductive paths interconnecting discrete components in multi-chip modules, or the like. Gold metal has particular applicability for applications requiring high reliability.
Gold metal powders are commonly used in hybrid microelectronic components where high reliability and high performance over an extended length of time are critical. Such uses include military applications, medical devices and aerospace applications. Typically, the gold metal powder is dispersed into a paste which is screen printed onto a circuit board and sintered to produce a conductive line. Gold metal has a higher reliability over extended periods of time than most other metals. Gold is also malleable and ductile. Gold does not substantially oxidize and therefore can be heated in air to temperatures up to its melting point. Gold is also resistant to corrosion in a variety of conditions.
Gold metal powder is typically dispersed into a thick film paste for most microelectronic applications. U.S. Pat. No. 5,039,552 by Riemer discloses an improved method for producing a gold conductor on a substrate in the manufacture of a thick film hybrid circuit. To improve adherence to the substrate, the gold conductor is formed from a thick film paste that includes the resinate of a metal that can form an alloy with the gold and that can also form an oxide at the firing temperature. Examples of such resinates include bismuth and cadmium resinate.
U.S. Pat. No. 5,167,869 by Nebe et al. discloses a gold thick film conductor composition which includes 75 to 95 percent gold wherein at least 90 percent by weight of the gold particles have an aspect ratio of no greater than 2. The paste further includes a cadmium borosilicate glass and a spinel forming metal oxide. The thick film paste is useful in electronic packaging for forming conductor patterns on ceramic based substrates, particularly patterns onto which metallic components are bonded by brazing. It is disclosed that the gold particles should be spherical in shape and should have a size of from about 0.4 to about 5 μm.
U.S. Pat. No. 5,429,670 by Miyoshi discloses a gold thick film paste for use in the manufacture of ceramic circuit boards. The paste includes 84 to 94 weight percent gold with an average particle size of 0.3 to 0.7 μm. The paste also includes vanadium oxide and copper oxide to improve adhesion of the gold conductor to the substrate.
Gold metal particles are also useful in dental compositions such as dental blended golds for application to a dental prosthesis. The prosthesis is formed from a base metal which is fired and produces an oxide on the surface of the base metal. A layer of gold is then applied to the dental prosthesis before application of the dental porcelain thereto. An example of this application is illustrated in U.S. Pat. No. 4,326,889 by Sperner. This patent discloses the use of spherical gold particles having an average particle size of from 0.8 to 1.2 μm.
Thus, there is a demand for gold powders having a small particle size for such applications. It would be beneficial if the powders had a narrow particle size distribution and spherical morphology. This demand is driven by the increasing complexity of the microelectronic components, including a reduction in size of the different components. However, methods for producing such powders on a commercial basis have not been disclosed in the prior art. Most methods for producing gold metal powders are based on liquid precipitation which can produce gold powders having impurities and having low crystallinity, among other problems.
U.S. Pat. No. 3,717,481 by Short discloses a method for the production of gold powder having a spherical morphology and a diameter in the range of 1 to 10 μm. The powder is produced by precipitating the gold from a gold chloride solution using a reducing agent. The gold powder must be washed to remove sulfite and sulfate ions produced by the process.
U.S. Pat. No. 3,725,035 by Short et al. discloses a process for producing precipitated gold powder by reducing a gold salt solution with a reducing agent in the presence of a protective colloid at an elevated temperature. The gold powder can include gold flakes, gold spheres or mixtures of the two morphologies. The powder must be washed with methanol and or water to remove impurities therefrom.
U.S. Pat. No. 3,768,994 by Daiga discloses a gold powder useful for the formation of thick film pastes. The gold powder is formed by dissolving a gold bearing material in aqua regia (HCl and HNO3) and adding an emulsifying agent to the solution. It is disclosed that the average particle size is less than about 20 μm.
U.S. Pat. No. 3,816,097 by Daiga discloses a multi component metal powder which includes gold. Examples include silver-palladium-gold mixtures. It is disclosed that the powder is substantially free of silver chloride or cyanide impurities.
U.S. Pat. No. 3,885,955 by Lutz et al. discloses a gold powder formed by the precipitation of chloroauric acid with ammonia from an aqueous hydrochloric acid solution and reducing the precipitate with a reducing agent. It is disclosed that gold powder can be produced having a spherical shape and an average size of from about 3 to 10 μm.
Silvert et al., in an article entitled “Synthesis of Monodisperse Submicronic Cold Particles by the Polyol Process”, Solid State Ionics Vol. 82, pages 53–60 (1995), disclose the production of gold particles having a size range of 0.1 to 0.4 μm. The gold particles are produced by the reduction of tetrachloroauric acid in a PVP-ethylene glycol solution. It is disclosed that gold particles having an average size of about 0.4 μm have an average crystallite size of about 47 nanometers.
Spray pyrolysis is not in common use for the production of metal powders containing small particles, such as those having a size of from about 0.2 μm to 5 μm. This is believed to be due to the high processing costs and low production rates associated with spray pyrolysis. Further, spray pyrolysis methods often produce hollow particles that are not sufficiently densified for most applications.
Generally, spray pyrolysis methods include the generation of liquid droplets wherein the liquid is a solution of a particle precursor. The droplets are then heated to evaporate the liquid, react the precursors, and form solid particles. U.S. Pat. No. 5,616,165 by Glicksman et al. discloses a method for making gold powders by an aerosol decomposition process. It is disclosed that substantially phase pure spherical gold powder can be produced by heating the aerosol solution to above 750° C. The gold powder is substantially spherical and has a wide particle size distribution.
It has also been proposed to form composite particles including gold metal. Composite particles can have enhanced properties, such as corrosion resistance or sintering resistance. For example, U.S. Pat. No. 3,966,463 by Fraioli et al. discloses metal powders including small amounts of refractory oxides to increase the oxidation resistance and sintering properties of the metal powders. In one example, it is disclosed that a gold metal powder was produced that included 0.5 weight percent titania. The powder is formed by a liquid precipitation route.
U.S. Pat. No.4,274,877 by Collier et al. discloses a metal-refractory composite powders having improved high-temperature properties. Examples include gold metal incorporating an alumina second phase.
U.S. Pat. No. 4,477,296 by Naier discloses a method for activating a surface of finally divided particles of conductive metals, including gold metal, using a thin surface coating of an oxide of the metal. The process includes treating the surface with an aqueous solution of a reducing agent, washing the particles and drying the particles to obtain the final metal product.
Despite the foregoing, there remains a need for gold-based metal powders having a combination of improved properties such as a small particle size, narrow particle size distribution, high crystallinity (large crystals) and spherical morphology. It would be particularly advantageous if such gold metal powders could be produced in large quantities on a substantially continuous basis.